Read pretty much any article here by Bret Contreras.
I don't know how long your limbs are, but if I get down on both knees and grab the bar, there isn't much range for me to actually perform a lift. So no, a kneeling deadlift would be a waste of time.
However, kneeling squats have been used by Westside lifters to improve squat and deadlift strength, and kneeling cleans and kneeling jumps (with and without weight) are used by many coaches to improve explosive hip action. So, this could be something you might want to look into.
The biggest and most obvious difference is the range of motion. Stiff leg deads have a longer range of motion, rack pulls are shortened to whatever height you set the bar. Also, stiff legs will have a bit more muscle activation, because of that added ROM.
As far as resting the weight at the bottom, some people can also do that with stiff leg deads, based on their flexibility and/or arm length, so that's not as significant an issue for everyone.